General Seismograph Info - Geometrics Forum

Is a Hammer Switch needed for good data quality?

Randl Rivera — Thu, 20 Mar 2025 22:12:07 +0000

If you are doing refraction and reflection then the trigger timing accuracy is important - especially when trying to get high resolution imaging.

For MASW the trigger timing accuracy is less important because the frequencies of interest are lower and the analysis method only relies on the phase differences of the signals between geophones and not the absolute time between shot event and the signal received on the geophone.

If you are acquiring individual records for each hammer blow and then inputting them into Pickwin, then a hammer trigger is not required, you can use an auto trigger method for the Geode. If you are stacking the data in the Geode, then a hammer trigger will be required.

Refraction and "other" modes explanation in SCS

Randl Rivera — Thu, 26 Oct 2023 17:19:39 +0000

Question:

Under the Geom>Survey Mode appears a window with "Refraction and Other." Before it was refraction and reflection. Now "other" means? Thanks for your help!

Thank you for reaching out. The manuals and technical documentation on our website and our new forum are good resources for questions regarding your hardware and software.

"When you are in Refraction Mode, the seismic software treats some of the data scaling operations differently. Autoscaling operations are done based on the amplitude of the noise before the first breaks to minimize the amount of manual scaling that may be necessary. In Reflection or Other Mode, trace autoscaling is done by normalizing each trace to the maximum value of the trace shown on the display.

Note: These are display features only – they do not affect the actual data. Refraction data acquired while in reflection or other mode, and vice versa, are perfectly valid."

What is “apparent velocity”?

Gretchen Schmauder — Fri, 01 Sep 2023 15:18:44 +0000

This is the velocity that is measured directly from the travel time plot, uncorrected for any refractor dip relative to the surface. The apparent velocity is equal to the true velocity only when the refractor is parallel to the surface. If the refractor is dipping, the apparent velocity measured in the updip direction will be higher than the true velocity, and that measured in the downdip direction will be lower. The true velocity, especially for small dips, is close to the average of the updip and downdip velocities, although not mathematically so.

What is a “velocity inversion”?

Gretchen Schmauder — Fri, 01 Sep 2023 15:16:43 +0000

A velocity inversion refers to the case when seismic velocity decreases, rather than increases, with depth. When this happens, energy refracts away from the normal:

This means that no information from the refracting interface returns to the surface, hence no information is gained about that interface, even its very existence.

This is demonstrated by the animation below. There are three distinct velocity layers, but Layer 2 is slower in velocity than Layer 1. The result is a travel time curve that indicates the presence two layers -- layers 1 and 3. Layer 2 is not sampled, so there is no evidence of its presence in the record. However, the presence of Layer 2 does impact the travel time plot by moving the crossover distance out farther than it would be otherwise. The result is that not only is Layer 2 not detected, but the depth to Layer 3 is incorrect -- it is calculated to be deeper that it is.

What is “reciprocity”?

Gretchen Schmauder — Fri, 01 Sep 2023 15:15:22 +0000

The Principal of Reciprocity states that the time required for seismic energy to travel between two points is independent of the direction it is traveling. In other words, if it requires time T to travel from source to geophone, it will require the same time T if the source and geophone are interchanged. While this may seem trivial, it is an important concept in seismic refraction. The reciprocal time is used in the depth calculation and it is an important QC check in the field.

What is an “air wave”?

Gretchen Schmauder — Fri, 01 Sep 2023 15:12:37 +0000

An air wave is simply the sound or acoustic wave created by the source, which by definition is what travels through the air rather than the ground. It is what you hear. In most cases, the seismic velocity of the near surface materials has higher than the velocity of sound in air. But in some cases the air wave is faster, and can arrive at the geophones before the seismic first break. This can make the first breaks difficult to pick.

An example of an air wave is shown below. The traveltime graph of an airwave is linear, and will be about 330m/s. It is generally lower in amplitude and higher in frequency than the first break, and can generally be dealt with in processing using a low-cut filter.

Environmental seismic refraction survey considerations

Gretchen Schmauder — Fri, 01 Sep 2023 15:03:20 +0000

Can I do refraction on pavement? It is exceedingly difficult to do this, because the pavement is generally has a much higher velocity that the materials below it. It is generally not recommended.

Can I do refraction on a hillside? Yes. The slope of the surface is not relevant (at least in theory, although there may be obviously practical limits).

Can I survey in the rain? Rain won’t hurt the seismographs. If you are using a StrataVisor or SmartSeis and it starts raining, they should be fine as long as they are kept vertical, with the screen pointing up. But acquiring data in the rain should be avoided, especially if there is lightning. Even in the absence of lightening, rain can be a significant source of noise, especially those raindrops striking the geophones directly. Obviously, the harder the rain, the more noise. If waiting the rain out is impractical, shielding the geophones themselves can reduce the noise significantly. If for some reason the seismic system must be left on the ground during an electrical storm, it is advisable to disconnect the trigger and geophone cables from the seismograph.

What is the Reflection Coefficient?

Gretchen Schmauder — Fri, 01 Sep 2023 15:00:48 +0000

The Reflection Coefficient R between two velocity layers is expressed as:

R = (ρ2V2 - ρ1V1) / (ρ2V2 + ρ1V1)

Where ρ = density and V = velocity. The quantity ρV is the seismic impedance of the material. The Reflection Coefficient is therefore the difference in seismic impedance over the sum of seismic impedance of two materials. From the above equation, it is apparent that R will be a positive number when V2 > V1, and a negative number when V2 < V1. A positive R means that the polarity of the reflected wave will be the same as that of the incident wave. A negative R means that the polarity of the reflected wave will be the opposite of the incident wave.

It should also be apparent that the larger the contrast in seismic impedance, the larger the amount of incident energy that is reflected (and the smaller the amount that is transmitted).

The above assumes normal incidence. For incident angles other than 90o, the equation is more complex.

What source should I use for seismic reflection surveying?

Gretchen Schmauder — Fri, 01 Sep 2023 14:55:31 +0000

The best source for seismic reflection is not always the most practical. Parameters such as cost, size, access, surface materials, and depth of penetration must all be taken into account. As always, a sledgehammer always supplies the most bang for the buck when practical. If the ground surface is too soft for a sledgehammer, you might consider a downhole seisgun. Small explosives are ideal in terms of portability and power, but for obvious reasons are often not feasible. Less portable sources like weight drops and vibrators should be considered when access allows and required depth of penetration exceeds that of a sledgehammer or seisgun. Although depth of penetration varies widely depending on the geology and cultural noise, you can expect to see somewhere in the range of 0.25 - 0.5 seconds with the latter two sources.

Which geophones should I use for seismic reflection?

Gretchen Schmauder — Thu, 31 Aug 2023 18:27:38 +0000

The de facto "standard" for high-resolution reflection is 40-Hz geophones. The 40-Hz cutoff will filter out much of the ground roll, allowing you to use the maximum gain even at near offsets, and taking advantage of the full dynamic range of the recording system.